The present invention pertains to the field of providing optical fibers having a Bragg grating. More particularly, the present invention pertains to a wrap (packaging) for such an optical fiber, a wrap that resists changes in the center wavelength of the Bragg grating in the optical fiber due to changes in temperature.
The use of fiber Bragg grating (FBG) devices in sensing applications has driven the requirements for very accurate measurements to be made on the center wavelengths of these devices, often down to 10xe2x88x9212 meters. Various instruments today are able to provide highly accurate readings of FBGs, especially when the FBGs function in a reflective mode. However, when FBG-based sensors are used in an environment where the temperature can change from room temperature to up to 150xc2x0 C. or higher, the spacing between the elements of the Bragg grating of the FBG device and the index of refraction of the fiber can change enough to cause unacceptable errors in the sensing application; the wavelength of light reflected by a fiber Bragg grating changes with temperature at a rate of approximately 10 pm/xc2x0 C.
Such changes in the reflected wavelength can be compensated for by measuring the temperature seen by the FBG device and applying a correction to the measured wavelength. However, it is often more advantageous to strain the fiber as a function of temperature in such a way as to compensate for thermal expansion and changes in index of refraction. This is accomplished by wrapping the optical fiber in a so-called athermal packaging, a packaging that adheres to the optical fiber, is stiffer than the optical fiber so that the optical fiber follows the change in length of the packaging, and that changes sufficiently over the required temperature range that the center wavelength does not shift more than an acceptable amount because of changes in temperature. This requires that the athermal packaging have a coefficient of thermal expansion of approximately xe2x88x928xc3x9710xe2x88x926/xc2x0 C.
Although FBG devices used in the telecommunications industry provide athermal packaging of use in temperature ranges encountered in the applications of that industry (typically from somewhat below 0xc2x0 C. to 80xc2x0 C.), there are applications where an FBG sensor sees temperature changes from room temperature or below to over 200xc2x0 C. What is needed is an athermal wrapping that will prevent the center wavelength of Bragg grating of an optical fiber used in an FBG device from changing more than an acceptable amount for changes in temperature to up to at least 150xc2x0 C., and in some cases to changes in temperature to up to 200xc2x0 C.
Accordingly, the present invention provides an apparatus and corresponding method for counteracting the tendency for the wavelength of light reflected from a grating, such as a Bragg grating, inscribed in an optical fiber to shift in response to a change in temperature of the span of the optical fiber having the grating, the method including the steps of: providing the span of the optical fiber having the grating; and wrapping a resin system about the span of the optical fiber, reinforcing the wrapping with a layer of contrahelically wound reinforcing fibers and further reinforcing the wrapping with reinforcing fibers disposed substantially parallel to the axis of the span of the optical fiber.
In a further aspect of the invention, the contrahelically wound fibers are contrahelically wound at a polar angle of 45 degrees.
In another, further aspect of the invention, the contrahelically wound fibers are also braided.
In yet still another, further aspect of the invention, all of the reinforcing fibers are KEVLAR fibers or all of the fibers are graphite composite fibers.
In yet even still another, aspect of the invention, the span of the optical fiber is provided with a bulge on either side of the span in which the grating is inscribed and the athermal packaging is molded over at least a portion of both bulges as well as the intervening span, thus creating mechanical resistance to slippage of the athermal packaging during expansion of the optical fiber in response to an increase in temperature.